1. Field of the Invention
The present invention relates to an image forming apparatus used in a laser printer, a digital copying machine, or a facsimile apparatus for assuring high image quality in accordance with the types of images to be recorded.
2. Description of the Related Art
Since a conventional image forming apparatus, such as a laser printer, cannot perform halftone recording in accordance with 1-dot recording, an image such as a character or line image having no gradation levels is simply binarized, and an image is formed using the resultant binary data. To the contrary, image formation of an image, such as a photograph having gradation levels, is performed in accordance with a systematic dither or area gradation method as a binarization method. In recent years, in a laser printer, there is provided a pulse width modulation method wherein the width of a laser drive pulse is set to be variable to improve reproducibility of thin lines and to allow gradation expressions (Japanese Patent Application No. 1-79571).
It is difficult to reproduce both an image such as a character or line image and an image such as a photograph having gradation levels with high quality even with the above pulse width modulation method. If an image write laser beam has a sufficiently small beam size, a very thin line can be reproduced with pulse width modulation. When an image having gradation levels is to be reproduced, however, only the same effect as in the area gradation method is obtained (although image quality can be improved by a degree corresponding to the size reduction).
If the size of the image write laser beam is almost equal to a pixel size, an image density can be controlled in accordance with pulse width modulation. This technique is suitable for reproducing an image having gradation levels, such as a photograph, but is not suitable for reproducing a very thin line such as a character or line image due to a decrease in image density.
Both an image having no gradation levels and an image having gradation levels cannot be simultaneously formed with high quality according to the conventional techniques, resulting in inconvenience.